It seems the Asset Store is kind of buggy at managing downloads for different versions of Unity. It looks like it's trying to import the Unity 2017.1 version of Playmaker (probably the last version you downloaded) into Unity 5.6. UnityEngine.ImageConversion was new in 2017.1 so wouldn't work in Unity 5.6.

In Unity 5.6.1 make sure you go into the Asset Store download manager and re-download Playmaker. This should get you the correct version of Playmaker for the version of Unity you're using. If you've previously imported the 2017.1 version, you may need to restart Unity before and/or after downloading and importing the new version.

Cool! Thanks for reporting this. I submitted a bug to Unity (Case 954335) and they were able to reproduce, so hopefully, they will fix it...

Hi.

I don't think this was solved. I have to work on 5.5.6 for a client and had the same issue when installing Playmaker. The error message lead me here and even after restarting Unity and redownloading from the asset store, I still couldn't use Playmaker.

Hopefully, I had an older project with playmaker 1.8.3 on it and could install it.